In wireless communication systems, quadrature amplitude modulation is used for transmitting the radio frequency (RF) signal. In most of the wireless systems, the information bearing base-band signal is in a digital format. The base-band signal is converted to analog form by using a digital-to-analog (DAC) converter. This analog signal is up-converted by modulating a high frequency carrier to make it suitable for transmission. The DAC and up-converter are the power and silicon-area consuming blocks in a typical transmitter. The complex design of these blocks is very time consuming, resulting in a high design cost. Design complexity, power consumption and silicon-area are the important factors in mobile wireless applications for determining the cost.
FIG. 1 is a block diagram of a typical transmitter used in a radio-frequency (RF) transceiver chip of wireless communication systems. Referring to FIG. 1, the information bearing baseband signal is a digital signal having ‘I’ and ‘Q’ components, which are referred to herein as I-data and Q-data, respectively. The I-data and Q-data, each having N bits (where N can be in the range of 1 to 20), are normally generated by a digital baseband signal processor. In the RF transceiver chip, the I-data and Q-data of the base-band digital signal are converted to analog form by using digital-to-analog (DAC) converters 101 and 102, respectively. The outputs of DACs 101 and 102 are filtered with filters 103 and 104, respectively, to remove out-of-band components introduced by DACs 101 and 102. Thereafter, the analog signals output from I filter 103 and Q filter 104 are fed into variable gain amplifiers (VGAs) 105 and 106, respectively, and then up-converted to a high frequency suitable for transmission by modulating a carrier frequency.
Many of the systems modulate the carrier as quadrature amplitude modulation (QAM) using a single-side band (SSB) mixer that is an analog RF block. In FIG. 1, a pair of mixers 107 and 108 is used. Mixer 107 up-converts the analog I-data, which has been filtered and amplified, using an I-clock output from divider 109. Mixer 108 up-converts the analog Q-data, which has been filtered and amplified, using a Q-clock output from divider 109. Divider 109 generates I-clock and Q-clock from a clock signal from oscillator 110 by dividing the frequency by two. Thus, generated I and Q clocks have half the frequency of the oscillator and they differ in phase by 90 degrees. The outputs of mixers 107 and 108 are combined using adder 111 that operates as a combiner to provide the SSB output. The output of adder 111 is amplified by RF driver amplifier (RF PA) 112. In low power systems such as UWB, Zigbee, RF IDs, usually, RF PA 112 is the final stage providing RF output for transmission. However, in the cellular and WLAN systems, the RF PA 112 act as pre-driver and provides output to an external power amplifier. A bandpass filter (BPF) 113 filters the output of amplifier 112. The signal output from BPF 113 is then transmitted using antenna 114.
Recently, another approach has been used to replace the analog up-conversion mixer and pre-driver in cellular applications such as GSM/EDGE. The quadrature baseband signal components ‘I’ and ‘Q’ are converted to polar signal having amplitude ‘A’ and phase ‘φ’. FIG. 2 illustrates a digital power amplifier user for amplitude modulation. Referring to FIG. 2, the amplitude part of the signal is used for amplitude modulation of the carrier using a digital power amplifier. In this case, a digital controlled oscillator (DCO) 201 generates the clock having frequency equal to the carrier frequency. The digital amplitude data bits 202 are used in digital ‘AND’ operation with AND gates 203 with the clock from DCO 201. This portion acts as RF DAC. The outputs of AND gates 203 drive the power amplifier transistors 204 that generates the required amplitude modulated output, which is coupled to an external power amplifier 206 and antenna 207 via a matching network 205. The amplitude modulated signal has undesired components also. The dominant one is the carrier itself that needs to be suppressed, and other out of band frequencies components exist in the prohibited band. A typical output power spectrum of such a transmitter is shown in FIG. 5. Referring to FIG. 5, the carrier has high amplitude and there are components at multiple of the carrier frequency. To avoid the out-of-band signal components, various schemes are used such as one described in a U.S. patent application publication no. 20060119493. These techniques increase the transmitter complexity. Moreover, for transmission, the lack of a single-side band output usually results in increase of power usage.